1. Field of the Invention
The present invention relates to an ion-type photoacid generator and a photosensitive polyimide composition. More specifically, the present invention relates to an ion-type photoacid generator containing a naphthol structure, a photosensitive polyimide composition using the photoacid generator, and applications of the composition.
2. Background of the Invention
In the semiconductor device industry, semiconductors and liquid crystal displays (LCDs) with high integration, high density, high reliability, and high-speed properties have been developed and widely used. For these purposes intensive research has been conducted for organic materials which have good processability and high purity. For the organic materials to be used in the semiconductor device industry, the materials should be thermally stabilized at a temperature of 200xc2x0 C. or above which is required in processes for manufacturing semiconductor devices.
Polyimide is one of the suitable organic materials for this purpose, because it has high heat resistance, good mechanical strength, low dielectric constant, high insulation, high planarity, low impurities and high reliability.
Polyimide is typically prepared by obtaining a polyimide precursor solution through polymerization of a diamine compound and a dianhydride compound in a polar organic solvent such as N-methyl pyrrolidone (NMP), dimethyl acetamide (DMAc) or dimethyl formamide (DMF). The polyimide precursor solution is coated on a silicon wafer or a glass substrate and cured by exposing to heat to form a polyimide film. Commercial polyimide for electronic devices is available in the form of a polyimide precursor solution or polyimide film. Polyimide for semiconductors is available in the form of a polyimide precursor solution.
FIG. 1 is a schematic cross-section of a semiconductor device in which a photosensitive polyimide composition according to the present invention is applied as a buffer coating film. In resin seal large scale integration (LSI), shrinkage of the resin after sealing and thermal stress by a difference in heat expansion coefficients between the resin and the chip can generate cracks on the passivation layer 5 of the chip or damage the wire bonding 2. These problems can be solved by using a polyimide to form a buffer layer between the chip and the sealing material 1. The polyimide layer should be thick to provide a good buffering effect, thereby improving the productivity of semiconductors. It is preferable that the thickness of the polyimide layer is 10 xcexcm or above.
With reference to FIG. 1, polyimide layer 6 has formed therein via holes, for example to facilitate linkage of electrodes and wire bonding pad 2. To form the via holes, a method of coating a photoresist on the polyimide layer 6 and then etching the polyimide layer has been widely used. However, recently photosensitive polyimides have been tried for application.
When a conventional non-photosensitive polyimide is used, an etching process is required to form holes by using a photoresist for wire bonding and connecting between metal wires. However, when a photosensitive polyimide is used, the lithography process using a photoresist can be omitted, and a buffer coating process can be reduced up to about 50%, resulting in high productivity and reduced manufacturing cost. Accordingly, intensive research has been conducted concerning the application of photosensitive polyimides to the semiconductor industry.
U.S. Pat. No. 3,957,512 to Kleeberg et al. discloses a method for preparation of relief structures by forming a film or a foil from a poly-addition or poly-condensation prepolymer. A photosensitive prepolymer solution is applied to a substrate to form a film, and the coated substrate is exposed to ultraviolet. On the exposed part, photopolymerization is performed to form a cross-linked structure. The unexposed part is dissolved or stripped off by an organic solvent and the relief structure which remains is annealed to remove the photosensitive organic radical during imide reaction. Finally a polyimide pattern is obtained.
U.S. Pat. No. 4,243,743 to Hiramoto et al. discloses a photosensitive composition comprising a precursor of a heat resistant polymer having carboxylic groups and a compound having a photosensitive olefin double bond and an amino or quaternary ammonium salt. The photosensitive polyimide of the patent is easily prepared and has less harmful byproducts.
Further, it has been known that positive-type photosensitive polyimides have more excellent properties than negative-type photosensitive polyimides. Generally speaking, positive-type photosensitive polyimides have better resolution than negative-type. And as the photo-irradiation area is relatively small in the positive-type, a better productivity is provided.
As the positive-type photosensitive polyimides use an alkali aqueous solution as a developing solution, it does not cause any serious problem in environmental pollution. However, the negative type photosensitive polyimides cause several problems in cost and environmental pollution such as wastewater disposal, because an organic solvent such as NMP or DMAc is employed. In spite of the above advantages of the positive-type photosensitive polyimides, they have not been commercially developed due to reasons of other technical difficulties.
Conventional techniques associated with positive-type photosensitive polyimides include a method of blending a polyamic acid as a polyimide precursor with a naphtoquinone diazide compound as a dissolution inhibitor which is patterned by the solubility difference between the exposed portion and the unexposed portion (Japanese Patent Publication Nos. 4-204945 and 6-73003), a method of blending a soluble polyimide having a hydroxyl group with a naphtoquinone diazide compound (Macromolecules, 23, 4796-4802, 1990), and a method of esterification of a polyimide precursor with an o-nitrobenzyl ester group as a photosensitive group (Japanese Patent Publication No. 60-37550).
The foregoing conventional techniques have various problems respectively. In JP Publication Nos. 4-204945 and 6-73003, it is difficult to obtain high resolution because the solubility difference is not sufficient. In Macromolecules (23, 4796-4802, 1990), polyimide precursors are limited in their structure, and physical properties such as transparency are poor. In JP Publication No. 60-37550, the sensitivity of the polymer is too low.
A photoacid generator is one of the important components of a photoresist using a chemical amplification process. Photoacid generators are classified as ion type and non-ion type. The ion type photoacid generators include an iodonium salt (JP Application No. 3-154059, a sulfonium salt and an ammonium salt (U.S. Pat. No. 4,069,055). The ion type photoacid generators are good for acid generation compared to the non-ion type generators, but have low solubility and poor stability. The non-ion type generator is an organic sulfonic acid ester, therefore stability is excellent, but acid generation is poor compared to the ion type generator.
Japanese Patent Publication No. 3-154059 discloses a photoacid generator having a dimethoxyanthracene structure that is applied to a photoresist for i-line. The photoacid generator has poor solubility and too high an UV absorption. Therefore if the generator is used in excess, the transparency is lowered.
Due to the above-mentioned shortcomings, it is difficult to obtain a pattern having a desired resolution when a prior photoacid generator is applied to a photosensitive polyimide, a photosensitive polybenzoxazole composition for a semiconductor passivation layer or a buffer coat layer, or a photoresist composition for build-up PCB (Printed Circuit Board).
Accordingly, the present inventors have developed a photoacid generator having a naphthol structure, which shows improved thermal stability, excellent solubility due to the hydroxyl group included in the naphthol structure, low absorption, and high acid generation efficiency. Further the inventors have developed a photosensitive polyimide composition prepared by using the photoacid generator, which can provide a positive type pattern having high resolution.
A feature of a preferred embodiment of the present invention is the provision of a photoacid generator with improved thermal stability, excellent solubility due to the hydroxyl group of a naphthol structure, low absorption and high acid-generation efficiency.
A further feature of a preferred embodiment of the present invention is the provision of a photoacid generator that is easily synthesized with high purity easily, and mass-produced economically due to the low prices of the materials to be used.
A further feature of a preferred embodiment of the present invention is the provision of a photoacid generator with high acid generation efficiency as well as low absorption, and a photosensitive polyimide composition which can make a pattern having high resolution to the extent that the aspect ratio is at least 2 on a film having a thickness of more than about 10 microns.
The above and other features and advantages of the preferred embodiments of the present invention may be attained by the detailed descriptions presented below.
In accordance with one aspect of a preferred embodiment of the present invention, a photosensitive resin composition is provided that includes a photosensitive resin and a photoacid generator.
The photoacid generator according to the present invention is represented by the general formula (1): 
wherein R1 and R2 are respectively H, OH or alkyl or alkoxy group of C1-5 and are the same or different, n is integer from 1 to 3, and Ar1 is a naphthalene unit.
The photosensitive resin useful in a composition according to a preferred embodiment of the present invention is represented by the general formula (2): 
wherein X is a tetravalent aromatic or aliphatic organic radical, Y is a bivalent aromatic or aliphatic organic radical, and R3 and R4 independently are H or a monovalent aliphatic organic protecting group removable by acid.